1. Field of the Invention
The present invention relates to analog signal samplers for imaging systems, and in particular, to analog signal samplers which use correlated double sampling (CDS) techniques for removing signal noise.
2. Description of the Related Art
Image sensors using charge couple devices (CCDs) are employed in many imaging applications such as scanners, digital cameras and video camcorders. As is well known in the art, CCD image sensors store analog signals as a series of discrete packets of charge. The charge stored at each cell of the CCD unit is proportional to the intensity of the light of an image incident on the area of that particular cell at a given point in time. The charge is transferred onto an output node once per sampling interval. Within each sampling interval, the previous charge at the output node is discharged to a circuit reference potential through a reset switch, thereby establishing a reference level at the output prior to each new charge transfer. Upon receiving a new charge, the output will charge to a new voltage level. A clock feedthrough appears at the output when the reset switch is turned off and the output settles to the reference level. However this reference level fluctuates due to the presence of noise having a mean square value given by kT/C. The existence of such noise limits the dynamic range of the analog image processing system. This problem is of particular concern in the more recently developed high resolution and high speed image sensors which must work with lower input charge and at much higher frequency.
One conventional technique for removing kT/C noise is that of correlated double sampling (CDS). This technique essentially measures the reference level of the output prior to the arrival of the new charge packet and subtracts this pre-charge level from the postcharge level. This method cancels the kT/C noise as well as suppresses low frequency noise.
Referring to FIG. 1, an early CDS circuit realization consists of an input buffer amplifier, a clamping circuit, a second buffer amplifier and a sample and hold stage. Following a reset pulse, when the CCD signal reaches the reference level, the clamp switch is closed briefly, thereby causing any reset noise to be stored on the coupling capacitor C.sub.CL. Following the new charge transfer, when the CCD signal settles at the new signal level, the sample switch is closed, resulting in an output voltage (across the sample and hold capacitor C.sub.SH) equal to the difference between the reference level and the new signal level of the CCD signal. Within one pixel, the reset noise is fully correlated between clamp and sample and is, therefore, virtually eliminated. The kT/C noise is more effectively removed if the coupling capacitor C.sub.CL is sufficiently large so that its own self-generated noise is small in comparison to the reference fluctuations on a much smaller capacitance of the floating diffusion node of the CCD.
Referring to FIG. 2, a circuit which eliminates the coupling capacitor C.sub.CL also eliminates the clamp switch. In this circuit, a delay line having delay dT, which is equivalent to the time difference between the clamp and sample pulse times in the circuit of FIG. 1, stores the reference level. A differential amplifier subtracts this stored reference level from the new signal level.
Referring to FIG. 3, an alternative circuit replaces the delay line and the differential amplifier with a delay line terminated by a short circuit. In this circuit, the CCD output signal is supplied to the input terminal of the delay line through a resistance Z.sub.O which is the same as the characteristic impedance of the delay line. The reflected delayed signal from the delay line is mixed with the non-delayed signal at the input of the delay line. With a proper delay time, the output signal from the sample and hold is the difference between the signal and reference level of the CCD signal.
By eliminating the clamping pulses, those circuits which use the delay lines are theoretically useful for high frequency applications. However, practical implementations of this approach suffer from complications resulting from signal loss and propagation dispersion. Furthermore, the delay line can not be realized in an integrated circuit (IC).
Referring to FIG. 4, another circuit which uses a differential average, or dual slope technique, theoretically yields the best signal-to-noise ratio for kT/C noise. The circuit consists of a buffer amplifier, a switch, an inverting amplifier and an integrator. The integrator input is connected to the output of the buffer amplifier or the inverting amplifier, or left open, depending upon the position of the three-position switch. First, the integrator is reset by closing switch position S4. Then, switch position S4 is opened and switch position S1 is closed during the reference level interval of the CCD signal, thereby allowing the inverted reference level to be integrated onto the integrating capacitor C during time t.sub.S. When the CCD signal changes to the image, or video, level, switch position S1 is opened and switch position S2 is closed, thereby connecting the integrator to the output of the buffer amplifier. The reference level plus the video level are integrated for the time t.sub.S when switch position S3 is opened. A signal equal to the difference between the reference level and video level is then held by the integrator. However, while this circuit has a better signal-to-noise ratio, it is less suitable for high speed applications due to the slow response of the integrator.
Since the CCD image sensor is generally the most expensive component in an imaging system, the lower cost contact image sensor (CIS) is seeing increasing use. The CIS consists of optical source, a rod lens array, photo sensors and electronic components housed in one unit. Unfortunately, the image signal produced by a CIS is different than the one produced by a CCD sensor. This presents a new challenge to the analog signal sampler which must work with both CIS and CCD sensors.